One problem which satellite communications systems address is the handing-off of communications between beams. Relative movement between subscriber units and beams causes the subscriber units and the communication links directed thereto to move between beams. To permit continuous communications in an ongoing call, a system must "hand-off" the communication when the subscriber unit crosses a beam boundary. If a communication is not handed-off to a new beam upon leaving an old beam, the communication will eventually be lost because the strength of signals over which communications take place would diminish to a point where the satellite cannot receive the subscriber unit's transmissions, or vice versa.
A satellite cellular communication system with non-geosynchronous orbiting satellites must address the motion of the satellite's antenna beam coverage areas relative to the subscriber units. The satellite beams sweep over the earth's surface at such a high rate of speed that a given subscriber will move through a number of beams during a particular call. In many cases, a subscriber unit may also be serviced by two or more different satellites during a call.
Typically, a conventional cellular communication system deals with the satellite motion by "handing-off" subscriber units from beam to beam or satellite to satellite when the subscriber unit detects that better service is available in a new antenna beam. Whenever one of these hand-offs occurs, the subscriber unit is assigned a new communication channel in the new beam. Each of these hand-offs requires the exchange of signaling data between the satellite and the subscriber unit. If these signaling messages are not received on time, the call may be dropped when the current beam no longer provides enough signal strength to maintain the link.
A hand-off from one satellite to another (inter-satellite hand-off) usually involves more system resources than a hand-off between beams on the same satellite (intra-satellite hand-off). In addition, synchronization is usually more difficult to achieve during inter-satellite hand-offs, which can lead to dropped calls, packet loss, and audio degradation. Therefore, a hand-off algorithm which minimizes inter-satellite hand-offs is desirable.
Another problem related to the hand-off process is known as the "ping-pong effect". This effect occurs when inter-satellite hand-offs occur closely spaced in time. One reason for the ping-pong effect is shadowing. When a given satellite is heavily shadowed for a short duration, an inter-satellite hand-off is undertaken to another satellite which is not heavily shadowed (faded) yet may be lower in received beam power. When the heavy shadowing (fading) on the original beam subsides, conventional hand-off algorithms will initiate a hand-off back to the original beam. This can occur when a subscriber unit moves behind an obstruction. Such a "ping-pong" effect is undesirable, especially from a system standpoint.
Thus, what is needed are a method and apparatus that provide a hand-off algorithm which minimizes inter-satellite hand-offs. Further, what is needed are a method and apparatus which significantly reduces the ping-pong effect.